Adds support for server-side filtering of types.

Types (formerly planning element types) are now filtered on the server.
This allows for some performance gain when not traversing work packages
(formerly planning elements) on the client unneccessarily. This speeds
up both the client side filtering process as well as network transfer.
Introduces start and end date generation in seeds.

Work packages previously had no start or end date. This is now fixed.
Also, type used to be settable via mass assignment, which is now broken.